Psaki said the apparent kidnapping of Simon Ostrovsky, a reporter for Vice News, would violate terms of a de-escalation deal reached last week in Geneva. She called on Russia to "use its influence" with the separatist groups to bring about Ostrovsky's release.

"We are deeply concerned about the reports of a kidnapping of a U.S. citizen journalist in Slovyansk, Ukraine, reportedly at the hands of pro-Russian separatists," Psaki said.

"We condemn any such actions, and all recent hostage takings in eastern Ukraine, which directly violate commitments made in the Geneva joint statement," Psaki said. "We call on Russia to use its influence with these groups to secure the immediate and safe release of all hostages in eastern Ukraine. We have also raised our concerns with Ukrainian officials as they work with local authorities to try to de-escalate the security situation in and around Slovyansk."

During a press conference on Wednesday, according to the Russian media outlet Gazeta.ru, the "People's mayor" of Slavyansk, Vyacheslav Ponomarev, said the pro-Russia militia had "taken" Ostrovsky. A representative for the militia confirmed to the Associated Press that the pro-Russian insurgents are holding Ostrovsky. When asked why, she said only that he was "suspected of bad activities."

A Vice representative said Tuesday that the publication was aware of the situation and in contact with the State Department.

"Vice News is aware of the situation and is in contact with the United States State Department and other appropriate government authorities to secure the safety and security of our friend and colleague, Simon Ostrovsky," the representative said.